1. Field of the Invention
The present invention relates to a method for measuring flow velocity of fluid, and more specifically, relates to an acoustic method for measuring flow velocity of fluid.
2. Description of Related Art
In current medical projects, ultrasound is applied to monitor organs and blood vessels in human body, and the Doppler Effect of ultrasound may be applied to measure the blood flow velocity of a portion of a certain blood vessel, which serves as important information to diagnose blood vessel choke and aneurysm.
FIG. 1A is a schematic diagram of a conventional method for measuring blood flow velocity in blood vessel. Referring to FIG. 1A, when blood vessel 10 is imaged using ultrasound, a sample volume S may be designated on a certain portion of blood vessel 10. Next, an ultrasonic beam 20a emitted by an ultrasonic transducer 20 may travel along the advancing direction E1, and travel through the sample volume S. The acoustic beam 20a may be scattered by the blood in blood vessel 10 and becomes ultrasound echo signal when passing through the sample volume S (not shown in FIG. 1A). After receiving the ultrasound echo signal, the ultrasonic transducer 20 analyses the frequency of the ultrasound echo signal to obtain the Doppler shift frequency, and the blood flow velocity V is calculated.
According to the above description, the conventional method for calculating the blood flow velocity V may be calculated by the following equation. Doppler shift frequency, and the equation of Doppler shift frequency is shown as the equation (1) below:
                              f          d                =                              2            ⁢                                                  ⁢            V            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                    λ                                    (        1        )            
In equation (1), fd is Doppler shift frequency, Doppler shift frequency, λ is the wavelength of ultrasonic beam 20a, and θ is Doppler angle which is the angle between the traveling direction E1 of the acoustic beam 20a and the direction of the blood flow velocity V. fd can be acquired via measurement, and λ is a given parameter.
The method of measuring the Doppler angle θ includes observing the blood vessel 10 image formed by ultrasound with naked eyes and manually marking a direction marker M in the sample volume S. The direction marker M is parallel to the blood vessel 10 shown in FIG. 1A. That is, the direction marker M represents the direction of the blood flow velocity V, and the Doppler angle θ is obtained.
However, the Doppler angle θ measured by the above method may be affected by manual observation and result in relatively bigger error. Secondly, the blood vessel 10 shown in FIG. 1A is a real blood vessel 10's two-dimensional image projected on screen. Therefore, actually the direction marker M might not parallel to the real blood vessel 10. Therefore, the Doppler angle θ measured using the direction marker M is prone to miscalculation on the blood flow velocity V. To address the above shortcomings, nowadays it is proposed to use Doppler bandwidth in Doppler spectrum to calculate the lateral component of the blood flow velocity V (i.e. V sin θ) to obtain the blood flow velocity V.
FIG. 1B is a Doppler spectrum graph based on ultrasound echo signal measured in the sample volume in FIG. 1A, wherein the vertical axis denotes the relative intensity of ultrasound echo signal, while the horizontal axis denotes the frequency. Referring to FIG. 1A and FIG. 1B, the Doppler spectrum SP is drawn through subjecting the ultrasound echo signal in the sample volume S to Fast Fourier Transformation (FFT). The frequency that the largest intensity value of the Doppler spectrum SP corresponds to is Doppler shift frequency fd, i.e. the Doppler shift frequency that is generated by the axis component V cos θ of the blood flow velocity V. When the relative intensity is a predetermined threshold T1, the frequency of the corresponding predetermined threshold T1 is the maximum Doppler frequency fmax. The Doppler bandwidth in FIG. 1B satisfies the following equations (2) and (3):Bd=2|fmax−fd|  (2)
                              B          d                =                              2            ⁢                          W              ·              V                        ⁢                                                  ⁢            sin            ⁢                                                  ⁢            θ                                λ            ⁢                                                  ⁢            F                                              (        3        )            
Wherein, Bd is Doppler bandwidth. In equation (3), W is the diameter of emitting source of the ultrasonic transducer 20, while F is the focal depth of the ultrasound acoustic beam 20a. W and F are given parameters, and the maximum Doppler bandwidth fmax and Doppler shift bandwidth fd can be obtained from FIG. 1B, therefore the lateral component V sin θ of the blood flow velocity V can be calculated from the equation (2) and (3), and the blood flow velocity V can be obtained.
However, the Doppler shift bandwidth fd in FIG. 1B may vary with the location of the sample volume S. More specifically, if the center O of the sample volume S deviates from the axis C of blood vessel 10, then the Doppler shift bandwidth fd may change, which results in error in calculation of the blood flow velocity V.